1. Field of the Invention
This invention relates to processes for producing chiral .alpha.-methylarylacetic acids. More specifically, it relates to a process for preparing (S)-.alpha.-methylaryl-acetic acids from mixtures, such as racemic mixtures, of (R)- and (S)-.alpha.-methylarylacetic acid esters by enantiospecific hydrolysis using extracellular lipases of microbial origin (EC 3.1.1.3).
2. Background to the Invention
A number of .alpha.-methylarylacetic acids (2-arylpropionic acids) are known as antiinflammatory agents: among the best known being ibuprofen, flurbiprofen, ketoprofen, and suprofen (all of which are substituted .alpha.-methylbenzeneacetic acids), and naproxen (a substituted .alpha.-methylnaphthaleneacetic acid). As is well known, the .alpha.-methylarylacetic acid molecule is chiral at the .alpha.-carbon atom and, therefore, exists in two stereoisomeric forms: the R- and S-forms (these forms are named by application of the "Sequence Rule", see J. Org. Chem., 35, 2863-7 (1970)). The S-enantiomers of these .alpha.-methylarylacetic acids generally possess greater antiinflammatory activity than the R-enantiomers ["Non-steroidal Antiinflammatory Drugs", J. G. Lombardino (ed.), John Wiley & Sons, New York, 1985, p. 3031]. For example, the S-enantiomer of 6-methoxy-.alpha.-methyl-2-naphthaleneacetic acid has 28 times greater antiinflammatory activity than the R-enantiomer [I. T. Harrison et al., J. Med. Chem., 13, 203 (1970)]. Hence, the S-enantiomer alone is used as the antiinflammatory drug naproxen (USAN and the USP Dictionary of Drug Names, 1986, p. 222).
The chemical synthesis of 6-methoxy-.alpha.-methyl-2-naphthaleneacetic acid [I. T. Harrison et al., J. Med. Chem., 13, 203 (1970)] leads to a racemic mixture of R- and S-enantiomers. Hence, resolution methods have to be employed to obtain the separate enantiomers from the racemic mixture. These resolution methods are, however, cumbersome and expensive. Generally, the chemical resolution methods entail the selective stoichiometric crystallization of a diastereomeric salt by the use of an expensive amine such as cinchonidine [P. Wirth et al., German Pat. (Offen.) 2,319,245 (1973); U.S. Pat. Nos. 3,787,580; 3,651,106; 3,906,0381] or dehydroabietylamine acetate [British Pat. 1,426,186 (1976)], or the use of a water soluble amine such as glucamine, which is difficult to recover [E. Felder et al., U.K. Pat. App. 2025968A (1980)]. Naproxen has also been prepared by the chemical resolution of a precursor using the expensive and less-available (I)-10-camphorsulfonic acid [G. I. Tsuchihashi, Tet. Lett., 5427 (1982)].
Various microbiological techniques have been reported for the preparation of chiral .alpha.-methylarylacetic acids. For example, the intact microorganism of Aspergillus sojae was used for the partial resolution of (.+-.)-6-methoxy-.alpha.-methyl-2-naphthaleneacetic acid methyl ester (S. Iriuchijima and A. Keiyu, Agri. Biol. Chem., 45, 1389 (1981)]. Unfortunately, the rate of conversion was very slow (only 16.3% of the substrate was converted), because the intracellular enzyme concentration was low and the amount of dried cells (400 mg) exceeded the amount of (.+-.)ester substrate (160 mg), and the R-isomer, rather than the desired S-isomer, of the acid was produced. Boehringer Mannheim GmbH, in European Pat. App. 153474, describe the enantioselective enzymatic hydrolysis of (.+-.)-6-methoxy-.alpha.-methyl-2-naphthaleneacetic acid (using, e.g., esterases from Aspergillus oryzae, Aspergillus flavus, Aspergillus sojae, and Bacillus subilis) to produce the R-isomer of the acid, leaving the S-ester. The S-ester is then hydrolyzed with an esterase from hog liver or Pleurotus ostreatus to give the desired S-acid. Montedison S.p.A., in European Pat. App. 195717, describe the enantioselective hydrolysis of certain (.+-.)-.alpha.-arylalkanoic acid esters to produce the S-acids, using a lipase from Candida cylindracea, a lipase which is exemplified in my prior applications.